This invention relates to Schmitt trigger circuits, and more particularly to MOS Schmitt trigger circuits.
A Schmitt trigger in MOS technology typically provides hysteresis by varying the voltage on the source of an input transistor. A control transistor is coupled between the source of the input transistor and ground to control the hysteresis. The control transistor causes the source of the input transistor to be at essentially ground potential when a logic high is received by the input transistor. Alternatively, when a logic low is received, the control transistor allows a control voltage to build up on the source of the input transistor. Because in a typical MOS Schmitt trigger the control transistor is in series with the output, it has been found to be desirable for the control transistor to be of relatively high gain so as to provide a relatively high current carrying capability. Even if the output is to sink a relatively small d.c. current, for speed purposes it is still desirable to have relatively large current carrying capability so as to be able to quickly discharge the capacitance on the output. With the control transistor being of relatively high gain, a relatively high current is required to build up the control voltage. Consequently it has been found to be desirable to turn the control transistor off when the input is a logic low, which is the case when the control voltage is to be developed.
Although current consumption is reduced by turning off the control transistor, control of the control voltage is diminished. The control voltage becomes too high for optimum speed as well as causing an increase in effective input capacitance. With a larger differential between the input voltage level and the control voltage, more charge is stored causing the increase in effective input capacitance. Also with a higher control voltage speed is reduced because the time required to reduce the voltage on the source of the input transistor is increased.
U.S. Pat. No. 4,063,119 Odell et al and U.S. Pat. No. 4,071,784 Maeder et al both show a Schmitt trigger with a control transistor (transistor T.sub.3 and transistor 19, respectively) which is not switched off when a logic low input is received. Consequently, in such a case, a trade off is required between optimizing speed with a relatively large control transistor and minimizing current flow with a relatively small control transistor.